How do I texture paint with glow?
It has nothing to do with the painting per se.

First, you need to paint the emission map of your shader :


Make sure to use black as blank color when creating the texture, so it doesn't emit light when nothing is painted.
I don't use the Principled BSDF's emission here because there's no strength input.

The glow itself comes from his viewport which is in rendered mode:



And using Eevee which has a bloom effect that can be seen in the viewport:



You can do the same with Cycles, except it doesn't have any SFX visible in the viewport, you would need to do it through compositing using a glare node in fog glow mode: